In many electronic applications, switches are used to control current flow in a system. In high voltage systems, such as lighting systems and electric machines, a multitude of switches can be used to enable and disable high voltage and high current paths during system operation. In safety-critical applications, such as elevators, and electric automobiles, and public transportation vehicles, however, the state of switching components is tested and diagnosed in order to detect and remedy different types of system failures. For example, if a failed switch is detected when an electric car is being turned on, a controller on board the electric car can prevent the car from starting, well as log a message in the car's memory noting the switch failure so that the car's manufacturer or mechanic can later be apprised of the source of the failure.
In systems where switches are connected in series, for example, in high-voltage systems, galvanic isolated circuitry is used to determine the state of each single switch because the connecting points of the switches can be at any potential in the system. Furthermore, isolated diagnostic capability is used in cases where the power components and the control components are not referring to the same ground potential, or are even isolated, such as the case in electric vehicles where the ground used to reference the electric motor and other high voltage machinery is isolated from the ground used to reference control circuitry and low power microelectronics.
In the case of high currents and/or voltages, mechanical switches, such as relays, are used because their conduction losses are smaller than the conduction losses of semiconductor switches. To detect a switching state for a relay, knowledge of whether the power contacts are open or closed is used. For example, in conventional isolated switch sensing devices, such as a force-guided contact relay, power contacts are mechanically coupled to a sensing contact that is opened and closed in parallel to the power contact. In some cases, however, this mechanical coupling increases the size and the complexity for manufacturing of a relay, especially if the relay is hermetically sealed for arc suppression.
In some systems, fuses are also used to control current flow. In the event of an overload condition, a fuse is blown, which creates an open circuit, thereby preventing further current flow. An overload condition in the system can occur due to abnormal functioning of some components, or can occur due to other reasons, such as heavy external loading. If a system is operated without regard to the state of a fuse, additional damage can be done to the system. What is needed are systems and methods for fuse and switch state diagnostics.